An ultrasound system has become an important and popular diagnostic tool due to its non-invasive and non-destructive nature. The ultrasound system can provide high dimensional real-time ultrasound images of inner parts of target objects without a surgical operation. The ultrasound system may operate in various image modes such as a brightness mode, a Doppler mode and the like to acquire the ultrasound images for diagnosis.
In the Doppler mode, the ultrasound system can provide a color Doppler mode image that visualizes velocities of moving objects (e.g., blood flow, heart, etc.) or scattered objects. The color Doppler mode image includes a power mode image visualizing powers of Doppler signals as two-dimensional (2D) distribution and a velocity mode image visualizing velocities of the moving objects, which may be computed from the Doppler signals, as 2D distribution. The color Doppler mode image not only visualizes the blood flow in real time but also represents a status of the blood flow at a wide range from the blood flow of a high velocity in a large vessel to the blood flow of a low velocity in a small vessel.
The Doppler signals may include low frequency signals (so-called clutter signals) due to motion of a cardiac wall or valve of a heart. The clutter signals can be an obstacle to accurately detecting velocities of the blood flow. Thus, the ultrasound system may employ a clutter filter to remove the clutter signals.
The clutter signals are typically distributed at the low frequency band and pure Doppler signals are typically distributed at a high frequency band. Thus, a high pass filter may be used to extract the pure Doppler signals. Since the clutter signals have higher amplitude than the Doppler signals, it is required to use a high pass filter of good performance to extract the pure Doppler signals.